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Geometric genus of a plane curve by delta invariants
Statement
Assume the Axiom of Choice, inherited through the normalization, delta, curve-topology and cohomological genus interfaces below. Let be algebraically closed and let be irreducible homogeneous of degree , defining an integral plane curve whose singularities are isolated. Then the genus of the normalization is the sum running over the finitely many singular points of .
Facts & Assumptions
Given: The Axiom of Choice and an algebraically closed field , an irreducible homogeneous form of degree , the integral plane curve with isolated singularities, its normalization , and the delta invariants of its closed points.
Under Choice, for the integral plane curve of degree one has and . (Arithmetic genus of a plane curve)
Under Choice, for an integral proper finite-type curve over the algebraically closed field with normalization , one has , the sum finite and supported on the singular points; the delta invariant is , and exactly at the regular points. (Arithmetic genus, geometric genus and delta invariants, Delta invariant of a curve singularity)
Under Choice, for the integral plane curve over the algebraically closed field the normalization exists, is finite and birational, and the geometric genus is defined as . (Normalization of an integral finite-type curve by gluing affine integral closures, Geometric genus of a singular curve)
Under Choice, if is an integral finite-type -scheme whose underlying space has chain dimension one, then a proper closed subset is a finite set of closed points, and every point other than the generic point is closed. (Proper closed subsets of a curve are finite)
The Axiom of Choice is assumed for the cited interfaces. (The Axiom of Choice)
Proof
By [F2], each is finite and nonnegative, vanishes at regular points, and the singular points form a finite set of closed points. Thus the correction sum is finite and has the stated support.
The arithmetic genus is known. Since is an integral plane curve cut out by the irreducible form of degree , [F1] gives .
Normalization formula. Applying [F2] to and solving for the genus of the normalization gives ; by [F3] this is the geometric genus and the normalization exists with the stated properties.
Conclusion. Substituting the value of step 1.2 into step 1.3 gives ; the sum is finite by step 1.1 and vanishes exactly when is smooth, in which case the normalization is an isomorphism and the formula recovers the plane arithmetic genus. Choice is inherited through [F1]–[F4].
Depends on
- The Axiom of Choice
- Delta invariant of a curve singularity
- Geometric genus of a singular curve
- Proper closed subsets of a curve are finite
- Arithmetic genus, geometric genus and delta invariants
- Normalization of an integral finite-type curve by gluing affine integral closures
- Arithmetic genus of a plane curve
Used by
Dependency tree · two levels
110 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- The Stacks Project, Algebraic Curves (tag 0BRV) (standard reference, not scraped)
- William Fulton, Algebraic Curves (Internet Archive copy), Chs. 6-8 (standard reference, not scraped)
- Ravi Vakil, The Rising Sea (version of October 21, 2025), Chs. 19 and 21 (standard reference, not scraped)